Method and circuit for echo cancellation in a telecommunication system with non linear transmission paths

0001In modern telecommunication systems, the voice quality is a essential quality feature. For example, For one participant, the Speech intelligibility, the sound, the naturalness, the completeness of the Voice communication and intercom features for a possibility subjectively perceived speech quality. Lens is the Transmission quality significantly disturbing echoes of the control determined that one by undesired coupling between Receive path and a transmit path occur and at times, the greater than 50 ms are to be perceived as very annoying. contact echoes for example, by electrical coupling between the transmit path and the receive path as line echo at 2 wire / 4-wire transitions of hybrids and / or by the acoustic coupling between Loudspeaker and microphone in a radio telephone, especially when For example, a sub-optimal speakerphone available at.

0002It is known that the effect of the echo by echo cancellers substantially eliminating, see R. Wehrmann among others.: A signal processing method for improving the voice communication a hands-free. The telecommunications engineer, Publishing Science and Life Georg Heidecker GmbH, Erlangen, Volume 48, October 1994 10/94 Page 27-28. With an echo canceller is the Transfer function of a real telecommunication system possible accurately modeled. The incident at the subscriber signal passes then both the real and the by the echo canceller simulated system, and then the output signal of the Echo canceller from the echo-affected signal of the real system subtracted, so that thus the echo is largely compensated.

0003Such echo cancellers are pulses with finite response filters, short FIR filters, realized, the effect of the precise determination of the is filter coefficients dependent. See DE-A-4,430,189th So must very be reliably distinguished between echoes and local noise, between echoes from distant reflectors and echoes from close located reflectors and between receiving waveforms sufficiently precise measurements allow, such as adequate strong broadband signals and unsuitable for measurements Receiving waveforms, such as sinusoidal tones duration.

0004Furthermore, circuits are known which damp so strong echoes, that they are no longer perceptible to a participant. For this purpose a Compander used, with the transmission signals above a setpoint to a uniform level of compression provided that they have the desired value, amplified and provided they fall below the setpoint, are damped. Here, the transmission characteristic of the compander from a except Coupling factor between the transmit path and receive path and the Noise certain control signal from the reception voice signal of remote speaker controlled. See DE-A-196 11548th

0005To the echo canceller to the use of a filter improve, it is known, in addition to a main filter a so-called employ background filter to the determination of the Filterkoeffiezienten to improve. Given by each filter are effected Signal differences between echobehaftetem signal and Filter output compared and from the Comparison result, a control signal for improved adjustment of the Filter coefficients derived, cf. EP-A-0872962.

0006The solutions described have indeed in linear Transfer functions useful when namely echo signal and Input signal of the digital filter correlate. However, they are for Telecommunication systems with nonlinear transmission lines unsuitable, which always arise when the transmission path Encryptor and / or highly compressed speech coder / decoder are, as in mobile systems according to the GSM (Global System for Mobile Communication) standard or the AMPS (Advanced Mobile Phone System) standard is the case. in order to Mobile networks a comparatively good voice quality as in fixed networks at achieve, measures need to echo cancellation or there Echo cancellation are performed. Although given Known Solutions used directly in the terminals of the mobile subscriber will, however, there are a considerable number of in use located terminals in which no or little effective Echo compensation measures or echo attenuation measures are provided and the retrofitting of such measures a would mean excessively high cost, so that only measures Network itself can be taken.

0007This results in the problem of providing a method and an A circuit arrangement for echo cancellation in a non-linear Telecommunications system, in particular in a mobile radio system according to To indicate to the GSM standard.

0008This object is described in a first patent claim A method and a described in the fourth claim Circuitry solved.

0009The essence of the invention is that known from a landline Echo cancellation circuits with nonlinear circuits from a Telecommunications network with Sprachcodiereinrichtungen for simulating the echo path combined and present there signals to control be used.

0010The invention will now be explained on an exemplary embodiment. In the show accompanying drawing<dl tsize="7" compact="compact"><dt>Fig. 1</dt><dd>a block diagram of a first variant of the Inventive circuitry and </dd><dt>FIG. 2</dt><dd>a block diagram of a second variant of the Inventive circuitry.</dd></dl>

0011In Fig. 1, the essential components of a mobile radio system are to the GSM standard, FIG. These include a radiotelephone 1, eng. Mobile station MS, a base station 2, eng. Base Transceiver Station BTS, a transcoder unit (36), Eng. Transcoder Rate Adaptation Unit TRAU, the exemplified here in a functional unit 3 for controlling the base stations, eng. Base station controller BSC, and having a Mobile switching center 4, eng. Mobile Switching Centre MSC. in the Following are the modules with their English Abbreviations referred. The MS 1 includes a speaker 13, a Microphone 14 and an encoder 11 for the language in the transmission direction, eng. uplink, and a decoder 12 for the language in the receive direction, eng. downlink. At the BTS 2 an uplink channel decoder 21 is one and a downlink channel encoder 22. The TRAU 36 are an uplink decoder 31, a downlink encoder 32 and now also a Echo canceller 34 and a downlink decoder 33. Notwithstanding the arrangement shown in Figure 1, 2 of the TRAU in the BSC 36, the 3 TRAU 36 also arranged between BSC and MSC 3 4 or in the MSC 4 be. The downlink decoder 33 is in the in the TRAU 36 existing downlink data stream 51 the loudspeaker signal <i>x</i>(<i>n</i>) Of MS 1 as an output signal <i>x ~</i> (<i>n</i> + <i>T</i>) Of the downlink decoder produces 33 wherein the speaker signal <i>x</i>(<i>n</i>) With respect to the output signal <i>x ~</i>(<i>n</i>+<i>T</i>) of downlink decoder 33 is delayed by the time T. With this output <i>x ~</i>(<i>n</i>+<i>T</i>) Can now be correlated to the part of the acoustic coupling 15 between the loudspeaker 13 and microphone 14 originating echoes are compensated with the echo canceller 34, wherein the echo canceller 34 can be realized by a FIR filter. Advantageously, the already in the TRAU 36 existing bfi (bad frame indicator) signal for improving the adaptation of the FIR filter be used. Thus, the calculation of the filter coefficients for example interrupted when from the bfi signal an uplink channel poor quality is displayed. Starting from the uplink data stream 52, it is possible with an uplink encoder, not shown here, in Structure substantially equal to the uplink encoder 11 in the MS 1, the replicate influence of non-linearities in the uplink channel and the refine echo cancellation. However, it should be noted that the Complexity of a coder is about ten times greater than the complexity of a decoder. So it makes sense for an affordable solution, the to model the influence of nonlinearities in the downlink channel and the Setting the echo canceller 34 of the bfi-signal of the uplink channel to make them dependent.

0012The advantages described are equally applicable to use of a Compander, which in the above-cited DE-A- 196 115 48 described, or when using the combination of a Compander with an FIR filter effect. In FIG. 2, the Circuitry using such Echo attenuator shown 35th Also in this case, it is possible the non-linearities of the uplink channel with known circuits, and existing signals to model.

0013The echo cancellation can be further improved if more Quality information, for example the transmission quality in downlink channel are available. Such information is in the TRAU 36 present when the future GSM AMR (Adaptive Multi Rate) - Encoders and decoders are used.

0014Basically it is also possible to use an echo canceller in the MSC 4 to arrange. In order to replicate the nonlinearities here for a to obtain echo canceller korrelierfähiges signal is a Derailleur from a downlink encoder and a downlink decoder required, thus a solution that considerably more complex than The inventive solution is.

0015Overall, the invention provides a cost effective solution for a Echo cancellation indicated on power level in mobile networks.